The present invention relates to an axial feed plasma spraying apparatus.
In conventional plasma spraying apparatuses, a spray material is typically fed into a plasma arc or a plasma jet generated in front of the nozzles, in a direction orthogonal to the plasma (i.e., via an external feeding method). In the feeding method, when the spray material has a small particle size and a small mass, the plasma arc or plasma jet repels the material before the material reaches the center of the plasma. When the spray material has a large particle size and a large mass, the material penetrates the plasma arc or plasma jet. In both cases, the yield of spray coating from the used spray material is problematically poor.
In recent years, demand has arisen for plasma spraying of a suspension material containing sub-micron particles or nano particles, or a liquid material of an organometallic compound. When the aforementioned external feeding method is employed, the yield of spray coating is considerably poor, impeding the use of these materials as spray materials, which is also problematic.
In order to enhance the density and adhesion of spray coating film, the speed of the spray material particles jetted by a plasma spray apparatus must be elevated. However, when the conventional external feeding method is employed, with increasing speed, the plasma arc or plasma jet repels an increased number of spray material particles before the material reaches the center of the plasma. Thus, the conventional feeding method is not suited for high-speed feeding.
One known method for solving the above problems is an axial feed plasma spraying apparatus, which is adapted to feed a spray material into a plasma generation chamber in a nozzle, and jetting of the molten spray material together with a plasma jet through a plasma jet jetting hole (see, for example, Patent Documents 1 and 2).
According to the methods disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-231498 and Japanese Patent Application Laid-Open (kokai) No. 2010-043341, the spray material is melted in a plasma generation chamber disposed in a nozzle. Therefore, the molten spray material is deposited on the inner wall of the plasma generation chamber, on the tips of the electrodes, or in the plasma jet jetting hole, thereby impeding stable and continuous operation. In addition, the products obtained by such a plasma spraying apparatus sometimes bear non-uniform deposits of such material.
Another problem is considerable wear of a nozzle, which is caused by jetting of a spray material through the nozzle at ultra-high speed, increasing wear of the jetting hole.
Also, the plasma generation chamber remains at high pressure because of the plasma gas fed into the chamber. Thus, when a spray material is fed into the plasma generation chamber, a spray material feeder receives back pressure. This imposes a particular pressure-resistant design on the material feeder.
Japanese Patent Application Laid-Open (kokai) No. Hei 7-034216 discloses a plasma spraying apparatus having a plurality of divided plasma jet jetting holes, which are disposed in parallel, so as to increase the area of the formed coating film. This plasma spraying apparatus also has the same problems as described in relation to the aforementioned known axial feed plasma spraying apparatuses.
Japanese Patent No. 4449645, Japanese Patent Application Laid-Open (kokai) No. Sho 60-129156, and Japanese Patent Publication (kokoku) No. Hei 4-055748 disclose plasma spraying apparatuses each having 2 to 4 cathodes and 2 to 4 counter anode nozzles in which plasma flames (also called plasma jets) provided through the anode nozzles are converged.
However, the plasma spraying apparatuses disclosed in this art still have a problem of considerably low yield of spray coating. The problem is caused by poor contact of the converged plasma flame with the sprayed material due to non-uniform damage of cathode nozzles and anode nozzles occurring during the course of spraying operation and due to lack of flow rate uniformity of working gases. This results in insufficient heat exchange and scattering of the spray material to undesired sections of the apparatuses.
Also, since a plurality of cathodes and anode nozzles are cooled, the apparatuses must be provided with a complex cooling path, leading to considerable energy loss of cooling water. In addition, maintenance of such cooling systems is very cumbersome and requires a long period of time.